Study On The Succession Of Green Tide Species In The Yellow Sea And The Generational Alternation Of Ulva Prolifera

Green tide refered to an abnormal marine ecological phenomenon in which macroalgae bloom or aggregate into a large area and float under appropriate hydrological conditions,with obvious seasonality.Because of the huge biomass,long drift path,wide influence range,and the special physiological structure of green tide algae,which could make it drift on the sea surface for a long time,green tide in the Yellow Sea had attracted the attention of countless scholars and the local government in the past ten years.At present,the hypothesis that Pyropia culture raft area was the main origin in the shoal area of northern Jiangsu had been widely recognized,and some scholars have proposed the origin of propagules,multi-point origin and subsidence origin.However,most of the existing researches were to explore the influence of the physiological conditions and environmental factors of green tide algae on the bloom of green tide.And there was little research on the bloom mechanism of the source of green tide in depth in the Yellow Sea.The connection between the floating green tide algae also needed to be studied.The research on the reproduction of the green algae Ulva species only stayed at the stage of basic life cycle research and spatial distribution changes,while flow cytometry and chromosome preparation technology were relatively mature,which could be more convenient to explore the green algae of the Ulva sp.in depth.The reproductive biology of green algae may provide a source control reference for the current scientific prevention and control of green tide.Therefore,this study started with the species composition and succession of the permanent green algae on the laver raft,combined the succession rule of the dominant population of green tide algae in the process of floating in the South Yellow Sea,and the generation structure ratio of Ulva prolifera.At the same time,the problem of meiotic period positioning of the sporophyte U.prolifera with sexual life history was solved.Combined with the above results,the species composition and succession of the source area of the green tide bloom and the floating process,as well as the reproductive strategy of green tide algae,were described.The main results are as follows:1.From November 2020 to April 2021,the succession of the dominant species of attached green algae and microscopic propagules showed the rule of “Ulva flexuosa –Ulva linza – Ulva flexuosa”,with a high degree of consistency.The propagules of U.prolifera had always existed in the sediments.The propagules of U.prolifera were only detected in seawater from the end of winter,and the proportion was much lower than that of Blidingia sp.and U.flexuosa.A small amount of low-temperature-adapted green algae,Ulva compressa,was detected on rafts and in seawater,and only existed in autumn and winter.Blidingia sp.was detected only on rafts,Ulva intestinalis and Ulva torta only in seawater,and Ulva meridionalis only in sediments.2.The microscopic propagules of U.torta were cultured for the first time in the seawater of the South Yellow Sea.And the size of the whole chloroplast genome was105,423 bp through complete chloroplast genome sequencing.The content of AT bases in the chloroplast genome was significantly higher than that of GC bases,which was74.8%.The genome encoded a total of 100 functional genes,including 70protein-coding genes,3 r RNA genes and 27 t RNA genes.U.torta and U.meridionalis were related by phylogenetic analysis.3.Combined with phototaxis experiments,chromosome number observation and flow cytometry,it was found that the germ cells(spores)released from sporophytes with asexual life history still maintained the same ploidy as the parent algae and were diploid,while the germ cells(spores)released by sporophytes with a sexual history are haploid.The meiotic stage of the sexual sporophyte U.prolifera was further locked in the stage of forming germ cells.4.In 2021,type 5S-I was the main type of attached U.prolifera,with less type5S-II and gametophytes with sexual life history.Only a small proportion of type 5S-III was detected in the raft area.In the floating stage,the type 5S-II U.prolifera had more advantages.5.In 2021,the main species of attached and floating U.prolifera were gametophytes with a sexual life history,followed by a small amount of asexual life history sporophytes.